As is well known, metal structures which must be immersed in electrolytes, such as iron or iron alloys in water, are subject to a significant problem of corrosion. This problem occurs due to the flow of local current through the electrolyte between localized cathodic and anodic portions of the immersed structure.
Accordingly, in the past, a number of systems have been developed to counteract this corrosion by making the metal structure to be protected part of an electrical arrangement which holds the structure at a predetermined electrical potential. This is accomplished by providing an electrode which is also immersed in the electrolyte and spaced apart from the structure to be protected. The structure and the electrode are then coupled to terminals of differing potentials of a direct current voltage source (typically a rectified alternating current voltage). Thus, the structure forms part of a circuit comprised of the voltage source, the electrode, the electrolyte, and the structure itself. This provides a predetermined polarity of potential to the structure to reduce the likelihood of the development of corrosion produced by local current flow.
If the structure is maintained at a negative potential, the system is known as a cathodic protection system. On the other hand, if the structure is maintained at a positive potential, the system is known as an anodic protection system. Of course, in either case, the electrode will have a polarity opposite to that of the structure. Further, in either case, it is desirable to maintain the structure at a predetermined potential to continue to prevent corrosion, since an improper potential level can reduce the effectiveness of corrosion prevention, and, in some cases, actually stimulate the corrosion.
For illustrative purposes, all discussion hereinafter will relate to a cathodic protection system wherein the structure to be protected is a cathode, and the electrode with which it cooperates is an anode. Of course, it is understood that the system discussed herein can readily be converted to an anodic protection system by reversing the polarities of the structure and the electrode.
To hold the structure at a desired potential level, e.g., at a predetermined negative potential for cathodic protection, it is necessary to sense the potential of the structure and make any changes necessary to the rectified alternating current source for correcting the current flow between the cathode and the anode to maintain the desired potential level. Typically, the sensing is done through the use of a reference cell (e.g., a standard Cu-CuSO.sub.4 half cell) which is also located in the electrolyte spaced apart from the structure. This reference cell is coupled to a measuring means such as a voltmeter. The measuring means is also coupled to the structure. Therefore, the measuring means can readily determine the potential of the structure. Since it is known that the potential of the structure has to remain at a certain level to be effective to prevent corrosion, the determination of the structure's potential allows control of the voltage applied between the anode and the structure which establishes the potential of the structure. Such control can be manual, or with an automatic adjuster for the rectified alternating current source coupled to the anode and cathode.
Although some prior art systems such as U.S. Pat. No. 4,080,272 issued to Ferry et al show the use of a plurality of anodes in a single protected tank, it is standard to use a single reference cell and a single control system for controlling the voltage applied to all of these anodes. Thus, the actual voltage applied depends on the measurement of potential at a single point of the protected structure. Although this is sometimes quite adequate, the inventor has found that in certain situations it results in significant problems.
For example, in water tanks, the upper portion of the tank (e.g. the upper bowl portion) is subject to greater coating damage due to winter ice formations than the lower portion (e.g., the riser or lower bowl portion). In conventional systems, if the reference cell for the anodes is located in the upper portion, the level of the applied rectified alternating current voltage is high to account for the measurements taken by the reference cell due to the above-mentioned ice formations. However, this same level of rectified alternating current voltage is also applied to the lower anodes in the riser portion even though the lower portion is not subject to the same large number of ice formations. Thus, the rectified alternating current voltage is excessive for the lower riser portion. This wastes electricity, and, in some cases, the excessive current provided in the riser portion can actually lead to coating disbonding.
On the other hand, if the reference cell were to be located in the lower portion of the tank structure, the extra current necessary in the upper tank portion due to the above-mentioned problem is not provided since the reference cell will not sense the need for it. Therefore, the degree of protection achieved at the upper level will, in some cases, be insufficient.
Another disadvantage of using only a single reference cell and control system is that the entire protection of the tank structure depends on these units. If any fault occurs in either the reference cell, rectifier, or the control circuitry, the tank protection will be completely lost.
Some attempts have been made in the past to overcome the above difficulties by inserting variable resistors in the separate anode feed cables to allow some manual adjustment of current flow to various portions of the structure. This method has been somewhat successful in cases of elevated water tanks with wet risers. The rectifier directly provides DC current to the tank bowl anodes, while a secondary circuit using a variable or fixed resistor connected to the rectifier limits the current to the riser anodes. However, this method cannot effectively compensate for changes in all parameters governing the protective current densities, and is not automatic. Also, the resistors are power consuming devices which serve to lower the system's efficiency.